1. Field of the Invention
The present invention relates to a solid electrolytic capacitor.
2. Description of the Related Art
A functional polymer solid electrolytic capacitor is being noticed because the functional polymer solid electrolytic capacitor is superior to frequency characteristics compared to another electrolytic capacitor. Recently, a surface-mounted solid electrolytic capacitor is developed, the capacitor having a capacitance of a few hundreds μF, having ESR (Equivalent Series Resistance) lower than 5 mΩ in 100 kHz frequency range and having ESL (Equivalent Series Inductance) of approximately 1 pH in 10 MHz frequency range. The solid electrolytic capacitor is suitable for a decoupling circuit of a power supply line to be coupled to a CPU (Central Processing Unit). A high capacitance capacitor, in which a plurality of solid electrolytic capacitors are stacked and coupled in parallel, is developed.
These solid electrolytic capacitors are surface-mounted capacitors suitable for a power supply line such as a personal computer, a server or the like of which speed and frequency get higher. An exterior of the solid electrolytic capacitor is, generally, molded with epoxy resin. There is, however, some defects with respect to the epoxy resin molding.
Transfer molding is, generally, used as an epoxy resin molding method. The epoxy resin is heated to more than 150 degrees C. and the epoxy resin is put into with a pressure of more than few atmospheres, in the method. A capacitor element is subjected to much stress. This results in increase of a leakage current and generation of electrical short circuit. The epoxy resin of high temperature breaks into between electrode foils of the capacitor element. And the characteristic is possibly degraded because of separation of a polymer of the capacitor element.
The epoxy resin used as the exterior mold includes filler for increasing a filling density of the epoxy resin. There are many voids on molecular level. This results in a defect with respect to humidity resistance. A package crack may be generated because of a heating during mounting, in a case of exterior cladding with the epoxy resin. It is necessary that the thickness is more than a given value in proportion to an exterior size in order to avoid the generation of the package crack. It is therefore not possible to obtain a super slim package.
And so, Japanese Patent Application Publication No. 2005-116713 (hereinafter referred to as Document 1) discloses a molding method as a method other than the transfer molding. In the method, a thermocompression tape where a single liquid epoxy resin is impregnated is pasted to a polymerized element. And the epoxy resin dissolved with heat is molded. It is, however, difficult to obtain humidity resistance because an exterior molding material is epoxy resin, with respect to a solid electrolytic capacitor manufactured following the art of Document 1.
And so, the capacitor element may be covered with a metal case. In this case, it is possible to improve the humidity resistance of the solid electrolytic capacitor. It is, however, necessary to improve the humidity resistance and reduce the ESR for purpose.